Method for conducting data communications with subscriber stations, and radio communications network for implementing said method

ABSTRACT

In a radio communications network, the time is divided into a succession of frames for conducting data communications with subscriber stations located within a coverage area of two central stations of the network. In a first broadcast phase of each frame, a first central station (AP 1 ) transmits first frame format information (BCH), which defines a first downlink phase of the frame (DL phase) and a first uplink phase (UL phase). In the same frame, a second central station transmits, on the same frequency, second frame format information (BCH), which defines a second downlink phase and a second uplink phase for the communication of subscriber stations (MS) with the second central station (AP 2 ), whereby the phases within a frame do not overlap.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on and hereby claims priority to GermanApplication No. 100 38 668.7 filed on Aug. 8, 2000, the contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to data communication withsubscriber stations in radio communications networks. To be moreprecise, the invention relates to the problem of joint use of radioresources by a number of central stations in a radio communicationsnetwork such as this.

[0004] 2. Description of the Related Art

[0005] Access methods (Multiple Access, MA) which define the right of anindividual station to access the medium are used in order to allow anumber of stations to access a shared transmission medium incommunications systems of any desired type. A distinction is in thiscase drawn as to whether the medium is subdivided in the time domain(Time Division Multiple Access TDMA), in the frequency domain (FDMA), inthe code domain (CDMA) or in the space domain (SDMA) between thestations.

[0006] If the joint use takes place in the time domain, there is aproblem in defining the time period during which one station hasexclusive access to the transmission medium, in order to avoidcollisions occurring. If a number of stations access the shared mediumin an uncoordinated manner, collisions can frequently occur as soon asmore than one station is transmitting at the same time. If a centralstation (for example a base station) monitors the time access for agroup of stations (for example mobile stations), collisions can beprevented for these stations.

[0007] If the aim is to supply an area which is larger than the maximumrange of a central station, a number of central stations must be used.These central stations are normally assigned different frequency bandsfor communication with subscriber stations which are within range ofthem, in order in this way to avoid collisions between the accesses bythe central stations. If a supply area is subdivided into cells whichare supplied by fixed-position stations (base stations), and basestations may use only a limited number of frequency bands, this resultsin a cellular radio system.

[0008] The number of frequency bands which are available for a radiocommunications system is limited. This means that there must be a numberof central stations in a radio communications network having a largenumber of cells, sharing the use of one and the same frequency band.Efforts are normally made to arrange the cells geographically such thatcells which use the same frequency band are sufficiently far away fromone another that radio signals which are transmitted in one of thesecells no longer interfere with communication in another which isoperating at the same frequency.

[0009] As the density of subscriber stations rises continuously and thecommunication traffic in the mobile radio networks rises in acorresponding manner, this principle reaches its limits, however, sincethe number of connections which can be handled simultaneously using agiven frequency band, and hence the number of subscribers who can becontrolled simultaneously in one cell, are limited. In order to increasethe transmission capacity, it would admittedly be possible to considerdividing a cell and assigning different frequency bands for each of thetwo resultant cell elements; however, this generally leads to theproblem of the frequency which is to be used in one of the newly createdcells already being used by another cell in the vicinity, which is notsufficiently far away to make it possible to preclude mutualinterference.

[0010] Similar problems can occur if it is impossible to supply all thelocations within a given cell with a radio signal of sufficientintensity to allow problem-free mobile communication. Since thetransmission power of a central station cannot be increased withoutrunning into the risk of causing interference in other cells that areoperating at the same frequency in the radio communications system, andthe transmission power of the subscriber stations, which are generallyoperated independently of the network, is restricted in any case, animprovement in the signal supply can be achieved only by “sharing” thecentral station between two different locations within the cell, withthe difficulty that the “station elements” at the different locationsmust coordinate their radio traffic in order to avoid interfering withone another.

[0011] One approach for solving the problem of coordination of basestations which use the same frequency band has been proposed in DE 19824 961 A1. In this known method, the transmission frame for one TDMAradio signal is subdivided into a number of containers, with onecontainer representing a specific number of time slots in the TDMAframe, and the containers each being allocated for use by different basestations. One base station is therefore not allocated all the time slotsin the frame but only a subset of them, which it can in each caseallocate on the basis of any desired known method for communication withterminals.

[0012] One such method is suitable for communication when the amount oftransmission traffic is constant, in particular for speechcommunication; however, there are problems in using this for datacommunication, where the requirements for the time response relating tothe transmission are generally less stringent but where considerableamounts of data frequently need to be transmitted in short time periods.The capability to make use of packets which remain free in a synchronouschannel for speech transmission for data transmission is restrictedsince each central station may have access only to those synchronouschannels which correspond to the containers allocated to it. It is oftenuneconomic, or impossible, to allocate additional containers to acentral station which has to transmit a large amount of data at shortnotice, since all the containers that are available within the frame arealready being used by one central station.

[0013] Modern mobile radio standards which are also designed for mobiledata communication, such as GPRS or HIPERLAN/2, no longer use theconcept of synchronous channels which are predetermined by fixed timeslots in a transmission frame and, instead of this, use frames whosestructure is not predetermined in a fixed manner, but which each containformat information which provides a connected receiver with informationabout the format of the respective current frame and which, inparticular, defines the location of uplink and downlink phases in theframe. In a system such as this, subscriber stations and a centralstation can each interchange different amounts of data from one frame tothe next; a specific predetermined fraction of the transmissioncapacity, which does not vary with time, is no longer reserved forcommunication with a specific subscriber station and, instead, eachsubscriber station uses from one frame to the next precisely thetransmission capacity which it requires, and transmission capacity whichis not required for time-critical services such as speech transmissionis completely available for asynchronous data transmission.

SUMMARY OF THE INVENTION

[0014] An object of the invention is to specify a method for datacommunication with subscriber stations in a radio communicationsnetwork, which allows the same frequency band to be used by a number ofcentral stations and in the process fully exploits the advantagesoffered by communications systems without a synchronous frame structure,such as HIPERLAN/2 or GPRS.

[0015] Since frame format information must be transmitted at the startof each frame in a communications system with a variable framestructure, in order to allow subscriber stations to identify thelocation of blocks which are intended for it within the frame, or thetime which is allocated to it for uplink transmission within that frame,it is possible without any difficulties to allow a central station totransmit format information which in each case leaves a part of the timeperiod before the start of the next frame without any allocation to onephase or the other in the frame. In this way, one or more time periodsremain free within the frame, and these can be used by a second centralstation, which is transmitted in the same frequency band as the first,in order itself to transmit frame format information and to use theseremaining time intervals, or at least a part of them, for transmissionto or from it.

[0016] In order to coordinate the two or more central stations which areusing the same frequency, a shared administration unit is expedientlyused, which allocates the uplink and downlink phases to the centralstations on the basis of their transmission requirement. This allocationprocess is in each case carried out in the same way as the transmissionof the frame format information from new from one frame to the next,thus allowing quick reaction to changing transmission requirements ofthe central stations and/or of the subscriber stations which arecommunicated with them.

[0017] In this case, it is expedient for the central stations to informthe control unit of their transmission requirement broken down on thebasis of priorities. This makes it possible for the control unit, whenallocating transmission time to the central stations,,to first of alltake account of the transmission requirement which must be satisfied asan essential condition for time-critical applications, and then toallocate the transmission time which still remains on the basis ofurgency. In contrast to the situation with the container method, acentral station which needs to transmit a large amount of data at shortnotice can thus use for this purpose not only a fraction of the frameperiod which is allocated to it as standard, but it is also grantedaccess to all the transmission time which is not yet being used forhigher-priority communications tasks within a given frame, by itself orby other central stations which are connected to that control unit.

[0018] There are various possible ways to share a transmission framewith a predetermined duration between two or more central stations. Afirst option is to allocate each central station a continuous timeinterval within the frame, formed of a broadcast phase in which thecentral station transmits frame format information which defines the useof the transmission time allocated to it, an uplink phase fortransmission from the subscriber stations to the central station, and adownlink phase for transmission in the opposite direction. Sharing inthis way has the advantage that it can be carried out with littlecomplexity, since it requires that the frame be subdivided into only asmall number of subintervals, which do not overlap. Since, however, theframe format information must be transmitted at the start of each frame,and the start of the time interval which is allocated to a centralstation may vary depending on the transmission requirement of theindividual stations, it is necessary for this purpose for the centralstations and subscriber stations to be able to process frames ofvariable duration.

[0019] This problem can be avoided by subdividing the transmissionframes in such a way that the broadcast phases of the central stationsfollow one another directly. This makes it possible for each centralstation to transmit frame format information at defined times with aconstant period to the subscriber stations which are connected to it,thus making it easier for the subscriber stations to monitor the frameformat information.

[0020] The administration complexity in the control unit can also berestricted in this case, by allocating each central station a continuoustime interval which in each case includes an uplink phase and a downlinkphase.

[0021] The subdivision of the allocated continuous time interval into anuplink phase and a downlink phase can be carried out autonomously byeach of the relevant central stations, since time collisions with theother central stations are precluded just by the allocation of thecontinuous time intervals.

[0022] In a further variant, broadcast phases and downlink phases of thecentral stations within one frame are in each case combined to form afirst continuous time interval, and the uplink phases are combined toform a second continuous time interval. This variant has the advantagethat the time delays which necessarily have to be complied with as aresult of the signal delay times when changing between an uplink and adownlink are shortened, so that the available transmission time is usedmore efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] These and other objects and advantages of the present inventionwill become more apparent and more readily appreciated from thefollowing description of the preferred embodiments, taken in conjunctionwith the accompanying drawings of which:

[0024]FIG. 1 is a block diagram of a radio communications system withone cell in which the method according to the invention is used; and

[0025] FIGS. 2-5 are examples of frame structures which are used for themethod according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals refer to likeelements throughout.

[0027]FIG. 1 shows a block diagram of a HIPERLAN/2 network as an exampleof a radio communications network in which the present invention can beused. The network has a number of radio supply areas, three of which,C1, C2, C3, are illustrated in FIG. 1. Each supply area in a HIPERLAN/2radio communications network conventionally has a so-called access point(AP), where access exists to the landline network, or a centralcontroller (CC) as a central station which communicates with a number ofsubscriber stations MT which are within range of its radio signals,switching connections between subscriber stations in the same supplyarea or in different supply areas, or to a landline network (which isnot illustrated). Supply areas which are adjacent to the centralstations communicate with their subscriber stations in respectivelydifferent frequency bands, so that communication within one supply areadoes not interfere with the adjacent supply areas.

[0028] The supply area c1 differs from the conventional supply areas C2,C3 in that it has two central stations AP1, AP2 which use the samefrequency band for communication with those subscriber stations MT whichare within range of their radio signals. The extent of the supply areaC1 thus corresponds to the combination of the regions which are in eachcase covered individually by the two central stations AP1, AP2. Acontrol unit, which is referred to as APC (for Access Point Controller)coordinates the transmission and reception activity of the two centralstations AP1, AP2. For this purpose, in each of the transmission frames,which each last for 2 milliseconds and are also referred to as MACframes, the HIPERLAN/2 standard signals its communication requirement tothe control unit APC which uses this information to subdivide the nextMAC frame between the two central stations, and transmits a message tothem which defines the time intervals within the next MAC frame whichare available to each of the individual central stations AP1, AP2 fortransmission and reception.

[0029] Parts of the transmission time may be permanently reserved forthe individual central stations, for example in order to transmitsignaling information which is essential for correct operation of thenetwork. It is possible for the control unit APC to take account ofthese reserved transmission times itself in the allocation of the timeintervals in a MAC frame, without the central stations themselvessignaling to it the requirement for this time. Alternatively, thepermanently reserved transmission time may also be requested from thecontrol unit APC from one frame to the next, together with the time forthe transmission requirement, which varies with time. In a situationwhere one central station has no subscriber stations to supply, thismakes it possible for that central station to temporarily interrupt theradio traffic with the subscriber stations in its supply area on therelevant frequency band, or to start such radio traffic only whenrequired. In a cellular radio communications system, this makes itpossible, for example, when the frequency band which is allocated to acentral station is overloaded, for that central station to temporarily“borrow” transmission time in a frequency band of an adjacent station inorder to supply subscriber stations to a greater extent than that whichis normally possible in a single frequency band.

[0030] A distinction is drawn between different priority levels wheninforming the control unit of the communication requirement. The highestpriority level is that for time-critical connections which have alreadybeen established, in particular for speech connections which (in orderto make it possible to ensure a satisfactory Quality of Service) areinstructed that the transmission bandwidth is available for them in eachMAC frame. The communication requirement of the central stationsresulting from connections of this type must invariably be satisfied bythe control unit. A lower priority communication requirement isassociated with data transmission connections in which fluctuations inthe amount of data transmitted in each frame and short-terminterruptions are tolerable to a certain extent. The communicationrequirement for connections whose establishment has been requested forthe first time by subscriber stations may be allocated to the same or toa different priority level. That proportion of the transmission time ina frame which has not been allocated by the control unit to connectionsin the highest priority level can be allocated on a proportional basisto the two central stations AP1, AP2 depending on their requirement.This makes it possible to achieve a high level of flexibility in thedistribution of the transmission time; if one of the stations suddenlyhas an increased requirement for transmission time, the entiretransmission time which has not yet been allocated in a fixed manner forconnections with a higher priority can be allocated to it from one frameto the next.

[0031]FIG. 2 shows a first example of a frame structure which can beused by the central stations AP1, AP2 in the supply area C1 for radiotraffic with their subscriber stations MT. The two central stations AP1,AP2 each alternately use the shared frequency band, with the continuoustime interval which is used by one central station having the structure,which is known per se, of a HIPERLAN/2 frame, in which a broadcast phasein which frame format information and other information which isrelevant for all the subscriber stations located in that cell istransmitted being followed by a downlink phase DL phase in which thecentral station transmits payload data to the subscriber stations, andan uplink phase UL phase in which the subscriber stations transmit tothe central station, as well as a random access channel RCH which isused inter alia, by subscriber stations to signal to the central stationthat they wish to set up a connection. In order to distinguish betweenMAC frames, the time period which is allocated to a central station inan MAC frame such as this is in this case referred to as the “stationframe”.

[0032] In contrast to the conventional MAC frames in accordance withHIPERLAN/2, the downlink phase and uplink phase are, however, shortenedin this case, so that the station frames which have been mentioned donot as standard fill the time period of 2 ms that is provided for oneMAC frame. The station frames of the various central stations canjointly fill an MAC frame; however, it is also possible to provide forthe time which is not required for transmission by any central stationnot to be allocated to any station frame.

[0033] While a first of the two central stations can always carry outits broadcast phase on the normal 2 ms cycle, it is possible with regardto the other central station for changes in the allocation of thetransmission time to the stations to make it necessary to shift thestart of the station frame that is allocated to it, hence resulting in adiscrepancy in the value of 2 ms in the time interval between twobroadcast phases. With the frame format shown in FIG. 2, it is thereforeexpedient for at least the frame format information which is transmittedby the second central station AP2 also to contain information for thesubscriber stations stating when the next frame format information willbe transmitted.

[0034] An alternative solution is for the change in the lengths of thestation frames which are allocated to the two central stations by one 2ms frame first of all to be restricted to a few tens of microseconds, sothat, when subscriber stations have received frame format information,they can assume from this that the next frame format information will bereceived after a time interval of 2 ms plus or minus this maximumpermissible change.

[0035] With the frame structure shown in FIG. 3, the two centralstations each transmit the frame format information in the broadcastchannel BCH immediately one after the other at the start of an MACframe. In this way, precisely in time with the MAC frames, eachsubscriber station receives the frame format information that itrequires in order to identify the start of the frame control channel FCHand associated control channel which are intended for it and from whichit can find out which of the data items which are transmitted in thesubsequent downlink phase are intended for it, and the time at which itmay transmit to the central station AP1 or AP2 associated with it in theuplink phase of the station frame.

[0036] In the frame format shown in FIG. 3, the capability to change theduration of the station frames which are allocated to the variouscentral stations from one MAC frame to the next is not subject to anyrestrictions.

[0037]FIG. 4 shows a modification to the frame structure shown in FIG.2, in which the individual central stations are not allocated anycontinuous time interval for transmission and for reception, but atransmission time interval (BCH, FCH, ACH, DL phase) for each centralstation is separated by a reception time interval (UL phase, RCH) foreach station by a time interval which is in each case allocated to theother station. If this frame structure variant is used, the control unitAPC not only decides on the proportion of an MAC frame which isallocated to each individual central station, that is to say the lengthof the station frames, but also defines the limits of the transmissionand reception time intervals within each station frame. One advantage ofthis variant is that the transmission time is used more effectively.This is because a guard time interval has to be inserted during thechange between transmission and reception, in which it is neitherpossible to transmit nor receive, with the length of this time intervalbeing governed by the signal delay time between the central station anda subscriber station which is at the most remote location in the cell.While four such changes occur in each MAC frame with the frame structureshown in FIG. 2, there are now only two with the frame structure shownin FIG. 4. A further advantage of the frame structure shown in FIG. 4 isthat the time interval between the associated control channel ACH forone central station and the uplink phase for this station is greaterthan that with the structure shown in FIG. 3. A subscriber station whichhas to transmit in the uplink phase to the relevant central stationthus, on average, has precisely the same amount of time with the framestructure shown in FIG. 4 as with a conventional HIPERLAN/2 frame, inorder to process the control information which is transmitted in the ACHchannel, for example the timing advance or power control. Everyconventional HIPERLAN/2 subscriber station is thus able to communicatewith a central station which is using a frame structure as shown in FIG.4.

[0038] The variable time durations of the phases of the frame structuresdescribed above may, of course, also be zero for individual phases andframes. In this way, by way of example, the first AP1 of the two centralstations may at times be allocated all of the transmission/receptiontime in an MAC frame when the second central station AP2 has no data totransmit or the only data which it has to transmit can tolerate aninterruption in the transmission for individual MAC frames, in order inthis way to allow fast transmission of large amounts of time-criticaldata by the first station AP1.

[0039]FIG. 5 shows a further modified frame structure, in which thesequence of the transmission and reception phases in the structure shownin FIG. 4 is distributed over two MAC frames, that is to say over a timeinterval of 4 ms. This structure also allows the allocation of longcohesive phases for uplinks or downlinks to one of the two centralstations AP1 or AP2, so that large amounts of data can be transmitted ina cohesive manner, and the theoretical transmission capacity of onefrequency band, which is 54 Mbps in a HIPERLAN/2 network, can be usedvirtually without any restriction for payload data transmission.

[0040] The frame structures described above may also, of course, be usedfor cells in which there are more than two central stations sharing afrequency band, by in each case inserting transmission and receptionphases for the other stations between those of the two stationsmentioned above.

[0041] Furthermore, the invention is not restricted to a HIPERLAN/2network, but can be applied to any desired radio communications networkswhich support dynamic definition of the frame format.

[0042] In particular, these radio networks do not need to have acellular structure; the principle of the invention can also be usedwithout any difficulties in wire-free LANs (Local Area Networks) andad-hoc networks (self-configuring networks). One example of an ad-hocnetwork standard where the invention can be used subject to certainextensions to the standard is the Bluetooth standard; even HIPERLAN/2 issubject to developments which are intended to lead to aself-configuration capability, in particular for applications in thedomestic field.

[0043] The invention has been described in detail with particularreference to preferred embodiments thereof and examples, but it will beunderstood that variations and modifications can be effected within thespirit and scope of the invention.

1. A method for data communication with subscriber stations (MT) in aradio communications network wherein the time is subdivided into asequence of frames, in which a first central station (AP1) transmitsfirst frame format information (BCH, FCH and ACH) in a first broadcastphase of each frame, which information defines a first downlink phase ofthe frame (DL phase), in which payload data is transmitted from thefirst central station (AP1) to the subscriber stations (MT), and a firstuplink phase (UL phase), in which the subscriber stations (MT) areallowed to transmit to the first central station (AP1), and in whicheach frame has at least one second broadcast phase in which at least onesecond central station (AP2) transmits second frame format information(BCH, FCH and ACH), which defines a second downlink phase (DL phase) anda second uplink phase (UL phase) for communication from subscriberstations to the second central station (AP2), with the uplink anddownlink phases being allocated to the central stations (AP1, AP2) by acontrol unit (APC) as a function of a respectively signaled transmissionrequirement.
 2. The method as claimed in claim 1, characterized in thatthe central stations (AP1, AP2) inform the control unit (APC) of theirtransmission requirements broken down on the basis of priority levels,fixed reservations or a combination of both.
 3. The method as claimed inclaim 2, characterized in that the allocation process in each case takesplace from one frame to one or more of the subsequent frames.
 4. Themethod as claimed in one of the preceding claims, characterized in thateach central station (AP1, AP2) is allocated a continuous time intervalin one frame, which continuous time interval comprises a broadcastphase, an uplink phase and a downlink phase for that station.
 5. Themethod as claimed in one of claims 1 to 3, characterized in that thebroadcast phases of the central stations follow one another directly. 6.The method as claimed in claim 5, characterized in that each centralstation is allocated a continuous time interval in one frame, whichcontinuous time interval comprises an uplink phase and a downlink phasefor that station.
 7. The method as claimed in claim 4 or 6,characterized in that the central station (AP1, AP2) subdivides thecontinuous time interval into an uplink phase and a downlink phaseindependently.
 8. The method as claimed in one of claims 1 to 3,characterized in that broadcast and downlink phases of the centralstations (AP1, AP2) on the one hand and uplink phases on the other handin each case form a continuous time interval.
 9. A radio communicationsnetwork having a number of central stations (AP1, AP2) for datacommunication with subscriber stations (MT), characterized in that atleast two central stations (AP1, AP2), which use the same frequency, areconnected to one control unit (APC), which allocates separate timeintervals for the uplink and downlink phases to each of them within onetransmission frame, as a function of a respectively signaledtransmission requirement.
 10. The radio communications network asclaimed in claim 9, characterized in that the central stations (AP1,AP2) are set up to send a message about their transmission requirementto the control unit (APC).
 11. The radio communications network asclaimed in claim 10, characterized in that the message is broken down onthe basis of the priorities of the data to be transmitted.
 12. The radiocommunications network as claimed in one of claims 9 to 11,characterized in that the radio communications network has a cellularstructure.
 13. The radio communications network as claimed in one ofclaims 9 to 12, characterized in that the radio communications networkis a wire-free LAN or an Ad-hoc network.